1. Field of the Invention
The invention concerns opto-electronic components and more particularly semiconductor electro-optical modulators.
2. Description of the Prior Art
These components are used in fiber optic transmission systems. For example, they form part of the transmitters in which they are used to modulate the power or the phase of an optical carrier wave. A power modulator associated with a laser source, for example in the form of a laser with built-in modulator, can be used to modulate the power. Phase modulators in an interferometer-type structure such as a Mach-Zehnder structure can also be used to modulate the power.
Electro-optical modulators are routinely fabricated on a III-V element substrate with a "p-i-n" structure, i.e. including in succession the substrate doped to obtain n type conductivity, an undoped active layer and a layer doped to obtain p type conductivity. The whole is reverse biased by an electrical control voltage modulated in accordance with the data to be transmitted.
To reduce the amplitude of the control voltage, an active layer having a "quantum well" structure formed of a succession of wells and potential barriers is generally used.
Modulators having this structure can be used as power modulators or as phase modulators. In the first case, the control voltage is essentially intended to modulate the absorption of optical power in the active layer. In the second case, the voltage modulates the refractive index of this layer.
In order to improve the performance of optical transmission systems, research is in progress on modulators capable of operating at very high bit rates, for example at 40 Gbit/s. It is therefore necessary to design modulators capable of operating at high frequencies whilst assuring the greatest possible depth of modulation of the optical wave, to allow correct detection at the receivers. The modulators must also be capable of modulating a carrier wave of high optical power, for example in the order of 10 mW.
The p-i-n structure mentioned above has two kinds of limitation. The first is due to a saturation phenomenon which reduces the extinction rate of power modulators if the input optical power to be modulated increases. A similar saturation phenomenon also occurs in phase modulators, in the form of a reduction in the depth of modulation of the index. In quantum well structures this saturation is the result of an accumulation of holes in the wells which reduces the electric field in the wells. This greatly reduces the bandwidth of the modulator if the optical power increases because of an increase in the time for which holes are trapped in the wells.
A second bandwidth limitation, occurring even at very low optical powers, is caused by stray capacitances of the structure employed. They are due in part to the capacitance of the p-i-n junction between the two electrodes of the component.
In order to improve the saturation power, it has been proposed to produce modulators using multiple quantum wells having confined voltage barriers. This solution increases the saturation power by reducing the time for which holes are trapped in the wells.
Another solution is to make modulators of very short length, less than 100 .mu.m, for example, but the improvement in bandwidth is achieved to the detriment of the depth of modulation.
It is feasible to increase the thickness of the active layer by increasing the number of wells constituting it to limit the stray capacitance due to the p-i-n junction. This solution does reduce the capacitance but it leads to an increase in absorption losses of the modulator in its transparent state. This drawback also occurs in the case of the multiple quantum well structure having confined voltage barriers previously mentioned.
An aim of the invention is to provide the optimum solution to the previous two problems.